Method for producing strips made of steel, in particular for producing cutting and machining tools having improved service life

ABSTRACT

In a method for producing strips made of steel, in particular for producing cutting and machining tools having improved service life, a preliminary strip is produced from a melt of a hardenable steel in a casting process and then rolled out into a hot-rolled strip and then, if necessary, annealed and cold rolled. The preliminary strip is produced in a horizontal strip casting system, wherein the melt is discharged from a feed vessel onto a cooled conveyor belt circulating over two deflecting rollers and is cast, with a killed flow and without bending, into a preliminary strip in the range between 6 and 40 mm, then rolled into hot-rolled strip having a degree of deformation of at least 50%, wherein the cooling rate on the top side and the bottom side of the preliminary strip is set differently for the purpose of residual solidification outside of the centre of the strip.

The invention relates to a method for producing strips of steel, in particular for producing cutting and machining tools with improved service life, according to the preamble of patent claim 1.

Such cutting and machining tools can for example be knife blades or sawing blades, which are used in the industrial or private area.

For producing cutting and machining tools usually a steel sheet made from carbon rich martensitic steel is used, which is characterized by a high hardness and abrasion resistance. Beside a relatively high carbon content of about 0.4 to 1.25 mass %, chromium is oftentimes added to the steel for further increasing hardness, wherein chromium acts as carbide former. Common for this application is for example that a steel 100Cr6 according to steel-iron-material data sheet has a chromium content of 1.5 mass %. When the steel is also to be stainless, chromium is added in amounts from 11 to 16 mass % according to DE 11 2010 004 925 T5.

The hardness of the cutting or machining tool results from a quenching and tempering process in which the tool is heated to austenitizing temperature, is then quenched and subsequently tempered. Beside an increased carbon content the alloy element chromium results in a decrease of the critical cooling rate and a significant increase of the hardness as a result of carbide formation.

Typical production methods for steels for cutting and machining tools are the ingot casting and continuous casting method. The coarse carbide precipitations known to occur in the ingot casting, however, lead to a diminished strength increasing effect compared to finely dispersed precipitated carbides so that a higher Cr content is required. A higher Cr content however decreases the notch impact toughness.

It is also generally known that in particular high-carbon-content steels with C contents significantly above 0.80% C, and high-carbon-content steels with carbide forming alloy elements (for example Cr and Mo and the non-carbide forming element Si in sum together above 1.5%) cannot conventionally reliably be produced as slag and without faults by means of the conventional continuous casting technique. The risk of failure of these steel tubes is disproportionately high. The reason for this is the insufficient ductility of the material during bending and alternating bending of the strip immediately after the casting, which can lead to the formation of cracks. This generally also applies to steel types with high mass contents of carbon of more than 0.80%.

For producing cutting tools it is known from DE 11 2010 004 925 T5 to use steels that are produced by vertical strip casting between rollers rotating in opposite directions and that are subsequently rolled out. Compared to ingot casting, vertical strip casting is thought to have the advantage that the primary carbides generated during solidification of the steel are significantly smaller than in ingot casting so that when sharpening knifes or razor blades the risk of edge cracks on the blade is significantly reduced. Also these steels are known to be produced by means of strip casting. The cast strip is subsequently hot rolled, annealed and then cold rolled to the demanded final thickness.

A disadvantage of the known production methods for steel strips for producing cuffing or machining tools is that due to the solidification the residual solidification in the ingot casting, strip casting and vertical continuous casting takes place exactly in the center in the region of the later blade. The problem is that blades for the industry and for home use predominantly have symmetric shapes and therefore the region for the blades is located in the core region, i.e., the center of the steel strips produced therefrom.

Tests have shown that increased shrinkage cavities (blow hole) as well as a coarse solidification structure with large precipitations (sulfides, carbides) usually form in this region during solidification.

The tests confirm that the presence of segregation regions and also a banded arrangement of the precipitations in these production methods, which at static tensile shearing and cyclical stress as they occur also in blades and saw edges, can be the starting point for cracks.

The centered arrangement of these residual solidification zones and the premature wear due to fractures, cracks and the like associated therewith, reduce the service life of the tools and requires premature replacement associated with the corresponding costs.

It is therefore an object of the invention to set forth a method for producing steel strips in particular for producing cutting and machining tools, which avoids the disadvantages of the known methods and influences the solidification microstructure during the production of the steel so as to achieve a significant increase of the service life of the cutting and machining tools produced therefrom compared to the known methods.

According to the teaching of the invention, a method for producing steel strips is provided in which a pre-strip is produced from a melt of a hardenable steel by casting and is subsequently rolled out into a hot strip and if required is subsequently subjected to an annealing and cold rolling process, which method is characterized in that the pre-strip is produced in a horizontal continuous casting facility, wherein the melt is applied from a feed vessel onto a cooled conveyor belt which is guided about two deflection rollers, and is cast under conditions of calm flow and without bending to a pre-strip in the range of between 6 and 40 mm, is subsequently rolled to a hot strip with a deformation degree of at least 50% and wherein the cooling rate at the upper and lower surface of the pre-strip is set different so that the residual solidification of the pre-strip takes place at an off-centered position.

The horizontal continuous casting method according to the invention is unconventional for the production of strips from which cutting or machining tools are produced and, compared to the so far known production methods for steel strips from which cutting tools are produced, has the advantage that on one hand the line of the residual solidification is no longer situated centered in the strip plane but off-center so that the region for the blade is located outside the region of the residual solidification and thus avoids the described disadvantages of the known methods in which the solidification region is centered. The service life and edge retention of the cutting tool is thereby significantly improved.

At the same time the high achievable cooling rate of the strip during the horizontal strip casting causes the precipitations, for example carbides, to be distributed very finely and homogenously, which has an advantageous effect on the edge retention and durability of the cutting or machining tools. Optionally the proportion of carbide formers such as chromium can be reduced, which lowers production costs.

As a result of the finer microstructure and the finely distributed precipitations the strength may also be significantly increased so that compared to the ingot casting method alloy elements such as chromium, molybdenum etc. can be saved and regarding the continuous casting the risk of failure due to crack formation can be reduced.

A deformation degree during hot rolling of the pre-strip of at least 50% is required in order to generate a most fine-grained and most homogenous microstructure. Depending on the thickness of the hot strip to be produced and the alloy composition, the deformation degree can be more than 70%.

The method according to the invention is therefore in particular suited for producing hardenable steel types for cutting tools, which at carbon contents of 0.80% in particular in combination with carbide forming alloy elements (for example chromium and molybdenum with the non carbide forming element silicone in sum over 1.5%) cannot be cast in continuous casting as slab without faults.

The method according to the invention has the further advantage that the position of the plane of the residual solidification of the pre-strip can be easily controlled by different cooling conditions on the upper and lower side of the pre-strip, wherein one side of the strip is for example cooled at an increased rate by means of water and the other side is for example cooled at resting air. For example the bottom side of the pre-strip can be achieved indirectly via an intensive water cooling of the bottom side of the conveyor belt, also referred to as caster belt, while the topside of the pre-strip is cooled at air.

Hot strips produced with the method according to the invention were tested with different alloy compositions according to the following table 1.

TABLE 1a alloy composition 125Cr1 C Si Mn Cr 1.25% 0.25% 0.35% 0.35%

TABLE 1b alloy composition 100Cr6 according to steel-iron material data sheet 200 C Si Mn Cr 1.00% 0.30% 0.30% 1.50%

TABLE 1c alloy composition bainitic steel C Si Mn Cr 1.00% 2.70%% 0.75% 0.50%

Table 1a shows a steel 125Cr1, which has a C-content of 1.25% in weight and Cr-content of 0.35% in weight.

In table 1b the steel has in particular a higher chromium content of 1.50%, whereas the steel in table 1c has a significantly higher Si-content of 2.70 weight % relative the other steels.

In all alloy variants a very fine-grained homogenous microstructure could be generated, wherein the position of the residual solidification in the strip plane could be set to be outside the center of the strip in a targeted manner, i.e. outside the position of the later blade.

The method according to the invention includes different cooling strategies for the topside and the bottom side of the pre-strip, wherein these are cooled differently. In a preferred embodiment the bottom side of the pre strip is cooled at an increased rate by a water-cooled conveyor belt (also referred to as caster belt), while the topside of the strip is cooled at a slower rate under a protective gas atmosphere. As a consequence the region of the residual solidification shifts out of the “geometric center” upwards and is located at a position about ⅓ from the topside of the strip (FIG. 1, left side).

A further advantage results from the high cooling rates in the near net shape casting method according to the invention, which lead to a finer microstructure while avoiding coarse carbides and blow holes, which in the conventional production route over the continuous casting facility or ingot casting may occur (FIG. 1, right side).

The demand of steel consumers to avoid blow holes, coarse precipitations, for example coarse carbides, in particular in the region of the blade can thus better be met with the horizontal strip casting according to the invention, by realizing different cooling strategies at the topside and the bottom side of the strip.

In a symmetric blade, the blade can be produced directly centered or in an asymmetric blade correspondingly on the opposite side so that the blade is not located in the region of the line of the residual solidification (FIG. 2).

In particular the off-center solidification of the cast strip makes it possible to avoid regions of residual solidification along with all associated disadvantages (coarse carbides, blow holes), while at the same time ensuring a centered position of the blade region of knifes relative to the geometric center (core). In the case of knifes with off-center position of the blade region on the other hand the position of the topside and the bottom side of the cast strip can be taken into account in a targeted manner in order to position the blade region outside the region of the residual solidification. The position of the blade relative to the zone of the residual solidification also does not change as a result of regrinding. The processability of the blade is also improved and the risk of fractures of the blade due to by blowholes or coarse precipitations is avoided.

In particular coarse chromium carbides cause a local destruction of the blade in the case of sudden stress on knifes and oftentimes require regrinding or a complex replacement, which is avoided in the method according to the invention.

Beside the use for industrial knifes, the use as saw blades for cutting wood and plastic is also possible. The high carbon contents and silicone contents enabled by the strip casting make it possible to forgo using hard metal blades in the border region of the saw toothing because the basic material due to its analysis already enables the application of a hard and wear-resistant layer. The homogenous microstructure with finely dispersed carbides achieves significantly higher service lives associated with less frequent replacements.

FIG. 3 schematically shows a device for producing a pre-strip with off-center solidification and subsequent hot rolling process for producing the demanded sheet thickness.

Prior to the hot rolling process the casting method is performed with a horizontal strip casting facility 1 consisting of a rotating conveyor belt 2 and two deflection rollers 3, 3′. Also shown is a side sealing 4, which prevents that the applied metal 5 runs off the conveyor belt 2 to the right hand side. The melt 5 is transported by means of a ladle 6 to the strip casting facility 1 where it flows through an opening 7 provided in the bottom of the ladle into a feed vessel 8. This feed vessel 8 is configured in the manner of an overflow container.

Shown are also the device K for intensive cooling of the bottom side of the upper run of the conveyor belt 2 and the complete housing 11 of the strip casting facility 1 with corresponding protective gas atmosphere.

After applying the melt 5 onto the rotating conveyor belt 2 the intensive cooling causes solidification and the pre-strip 9 is generated, which at the end of the conveyor belt 2 is fully solidified to the most part. The cooling device K allows influencing the cooling according to the invention in a targeted manner relative to the topside of the pre-strip 9 so that an off-center solidification (metallurgical center) of the pre-strip 9 is achieved.

For temperature compensation and tension reduction a homogenization zone 10 follows the strip casting facility 1. The homogenization zone consists of the heat insulated housing 11 and a here not shown roller conveyor.

The following first stand 12 is either only configured as a pure driver unit optionally with a small pass or as rolling aggregate with a predetermined pass.

Following is an intermediate heating, advantageously in this case as inductive heating for example in the form of a coil 13. The actual hot forming occurs in the following stand array 14, wherein the first three stands 15, 15′ 15″ cause the actual pass reduction, while the last stand 16 is configured as smoothing stand.

Following the last pass is a cooling zone 17 in which the finished hot strip is cooled to reeling temperature.

Between the end of the cooling path 17 and the reel 19, 19′ a cutter 20 is arranged. This cutter 20 has the purpose to cut the strip transversely as soon as one of the two reels 19, 19′ is fully wound up. The beginning of the following hot strip 18 is then guided onto the second released reel 19, 19′. This ensures that the strip tension is maintained over the entire strip length. This is in particular important when producing thin hot strips.

Not shown in the Figure are the components of the facility for the optional annealing and cold rolling of the hot strip.

LIST OF REFERENCE SIGNS

No. designation  1 strip casting facility  2 conveyor belt 3, 3′ deflection roller  4 side sealing  5 melt  6 ladle  7 opening  8 feed vessel  9 pre-strip 10 homogenization zone 11 housing 12 first stand 13 induction coil 14 stand array 15, 15′ 15″ rolling stand 16 smoothing stand 17 cooling path 18 finished hot strip 19, 19′ reel 20 cutter K cooling device 

What is claimed is: 1.-13. (canceled)
 14. A Method for producing strips of steel, in particular for producing cutting or machining tool with improved service life, comprising: producing a pre-strip having a thickness between 6 and 40 mm under calm flow conditions and without bending in a horizontal strip casting facility by casting a melt of a hardenable steel from a feed vessel onto a cooled conveyor belt circulating about two deflection rollers; cooling a topside and a bottom side of the pre-strip at different cooling rates so that a residual solidification of the pre-strip is situated outside of a center of the pre-strip; and rolling the pre-strip into a hot strip with a deformation degree of at least 50%.
 15. The method of claim 14, wherein the bottom side of the pre-strip is cooled at a higher rate than the topside of the pre-strip.
 16. The method of claim 15, wherein the bottom side of the pre-strip is cooled indirectly by cooling a bottom side of the conveyor belt.
 17. The method of claim 16, wherein the bottom side of the conveyor belt is cooled with water and the topside of the pre-strip is cooled at resting air or in a protective gas atmosphere.
 18. The method of claim 14, further comprising after a complete solidification of the pre-strip and prior to a further processing of the pre-strip, passing the pre-strip through a homogenization zone.
 19. The method of claim 18, wherein the further processing is a cutting of the pre-strip into plates.
 20. The method of claim 19, further comprising after the cutting into plates, heating the plates to rolling temperature and rolling the plates.
 21. The method of claim 18, wherein the further processing is a coiling of the pre-strip.
 22. The method of claim 21, further comprising after the coiling, de-coiling the pre-strip, heating the pre-strip to rolling temperature and rolling the pre-strip.
 23. The method of claim 21, further comprising re-heating the pre-strip prior to the de-coiling.
 24. The method of claim 14, further comprising after the rolling, coiling the pre-strip, wherein the pre-strip is subjected to the rolling in-line.
 25. The method of claim 14, wherein the deformation degree is >70%.
 26. The method of claim 14, further comprising after the cooling, cold rolling the pre-strip. 